Processing, May 2018

Special Section Motors Drives N e w V F D t e c h n o l o g i e s i n harmonic mitigation Three level active front end drives open a new chapter in power conversion devices By Garrett Abbott Frey Schneider Electric armonics in electrical systems has been a major concern for engineers and designers since the invention of inverter technologies Since resources and energy are H limited system suppliers have become increasingly focused on the efficient use of variable frequency drives VFD including the mitigation of harmonics generated by the VFDs in their systems As the search for the optimal harmonic mitigation technology has introduced technologies such as multipulse rectification and passive active filters to deal with the issue of harmonics the three level active front end technology used in the Altivar 680 and Altivar 980 Low Harmonic AFE Drive Solutions from Schneider Electric offers harmonic mitigation performance in a small footprint allowing for more flexibility in numerous applications It is an honor to share more about this technology as a 2017 Processing Breakthrough Product of the Year The issue of harmonics To understand harmonics the types of loads in variable frequency applications linear and non linear must be understood first A linear load is where the current waveform has the same shape as the supply voltage A non linear load is where the current waveform changes with the supply voltage Non linear loads are common in all industries and are often caused by equipment such as welding machines induction furnaces power supplies and of course VFDs VFDs have input rectifiers which produce an input current that is non sinusoidal These currents can be represented by a sinusoidal fundamental current and additional sinusoidal harmonic currents A high level of harmonics can affect the efficiency and performance of other electrical devices connected to the same input power supply Power distribution systems typically include various types of power components such as reactors and transformers As current flows through these electrical components each component introduces a level of impedance With the introduction of impedance in the system a certain level of voltage drop is expected This voltage drop is directly proportional to the total amount of current flowing through the given system To improve power quality in the electrical systems many types of harmonic mitigation architectures can be deployed These non linear loads are the source of harmonic currents which cause voltage distortions and power quality issues that impact each electrical component in the system disrupting the control infrastructure supporting processes This can result in unwanted equipment behavior and even power loss Harmonic content in power distribution systems can lead to undesired outcomes such as disturbances in communications increased operational expenses and loss of revenue In addition some system loads drawing harmonic current often draw currents that result in increased ground currents and or induce common mode noise This can cause increased motor bearing temperatures in three phase induction or permanent magnet motors Both of these conditions can negatively impact the service life expectancy of the equipment connected to the power grid This article looks at how the three level active rectification technology works to mitigate these harmonics Three level active rectification technology By replacing the typical six input diodes with insulated gate bipolar transistors IGBTs such as the ones used on the output of most drives the input current wave shape can be controlled This is where the active part comes into play Three level active rectification is used to improve input current waveforms This is achieved by the addition of three bidirectional IGBT switches connected to the DC bus This three level topology also uses an additional common mode CM filter stage The CM filter stage allows for high efficiency and behaves like a passive rectifier circuit in terms of CM voltage To generate sinusoidal input currents in phase 22 Processing MAY 2018 Harmonic content in power distribution systems can lead to undesired outcomes such as disturbances in communications increased operational expenses and loss of revenue Schneider Electrics Altivar 680 low harmonic process drive All graphics courtesy of Schneider Electric

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